368 LUBRICANTS fiable residue has the specific rotation + 15° to + 16° or, very occasionally + 10°, and the iodine number is not below 55. Mixtures of mineral oil with the unsaponifiable matter of wool fat give intermediate rotations and iodine numbers. When, however, the unsaponifiable residue contains resin oils, the rotatory power and iodine number no longer give an indication as to the presence of wool fat since these substances also absorb iodine and rotate ; in such case, the very high specific gravity of resin oils (0-97-0-99) must be borne in mind. 8. Detection and Determination of Free Lime and other Mineral Substances.—About 10 grams of the lubricant are treated for 15 minutes in a reflux apparatus with 50 c.c. of benzine and 5 c.c. of alcohol, the insoluble residue being coUected on a filter, washed with the benzine-alcohol mixture, weighed and examined by the ordinary methods to see if it contains lime, calcium carbonate, barium sulphate, talc, graphite and other mineral sub- stances. * * * Stiff lubricants vary in composition according to the uses to which they are to be put. Those for stuffing-boxes in steam-engine cylinders are composed of a solid fat (tallow) or of a mixture of tallow with wax and oil. Those for ropes contain solid fats, wax, oil, talc, etc., and those for the chains of cranes, lifts, etc., are similar. Lubricants for rolling mills should melt above 100° ; some are composed of pitch from fats or mixtures of this with crude petroleum pitch, while those with a basis of wool fat consist of partially saponified wool fat, with or without resin or acid resin oil. Briquettes of vaseline, also used for rolling mills, are formed from mineral oil and soda soap. Lubricants for gearing are composed of a stiff fat with graphite or talc ; oil, tar, resin, wax, paraffin wax and ceresine may also be added. Lubricants for maintaining the flexibility of belting consist of fish oils mixed with a solid fat (tallow, wool fat, wax). Adhesive lubricants for belting con- tain, besides these fats, resin, resin oil, wool fat, etc. Lubricants for the axles of vehicles usually contain tar oil or resin oil in place of mineral oil and mixed lime and resin soaps in place of lime soap and fatty acids. 2. Emulsive Lubricants These are colourless, yellowish or reddish, and often fluorescent liquids, which are mixed with water to form a kind of emulsion ; they are some- times sold ready emulsified and then have the appearance of milky liquids. Besides observing the colour, transparency and odour, and determining the flash point, viscosity and behaviour at low temperatures (see Heavy Oils, Physical Tests, 5, 7 and 8), the following tests are made on these oils (see also Turkey Red Oil, Chapter XI). 1. Emulsivity.—When shaken with water in any proportion, the oil should give a milky emulsion, which should not separate oily drops at its surface even after a long rest. When left overnight at the ordinary tem- perature, the emulsion of 5 grams of oil with 100 grams of water should undergo no change or should at most deposit yellowish caseous flocks. 2. Determination of the Water and of the Volatile .Solvents.—The emulsive oils may contain volatile solvents (alcohol, benzine), which are recognised by the smell or, better, by distilling the material on a water-bath and examining the distillate. The amount of volatile solvents and water, while the liquid remains clear.r.